Method for operating a steam turbine with an impulse rotor and a steam turbine

ABSTRACT

A steam turbine ( 10 ) has an impulse rotor ( 11 ), a multiplicity of stages (N, . . . , N+4) which are arranged in series along a machine axis ( 12 ) and with each of which is associated a wheel disk ( 11   a - e ) equipped with corresponding rotor blades ( 17   a - e ). The rotor blades ( 17   a - e ) of the individual stages (N, . . . , N+4) project into a common axial steam passage ( 15 ) into which steam ( 14 ) enters via an inlet duct ( 13 ), and diaphragms ( 16   b - e ) are arranged between the stages (N, . . . , N+4) in each case. A simple and effective protection of the wheel disks against stress corrosion cracking is achieved by dry superheated steam ( 14 ) being fed to the steam turbine ( 10 ), and for reducing or for avoiding stress corrosion cracking on wheel disks ( 11   b - e ) of the impulse rotor ( 11 ) which are exposed to the risks of wet steam, by the dry superheated steam ( 14 ) being used for purging of the wheel disks ( 11   b - e ) of the impulse rotor ( 11 ) which are at risk.

This application claims priority under 35 U.S.C. §119 to German application no. 10 2010 012 583.0, filed 23 Mar. 2010, the entirety of which is incorporated by reference herein.

BACKGROUND

1. Field of Endeavor

The present invention relates to the field of steam turbines. It relates to a method for operating a steam turbine and to a steam turbine useful for implementing the method.

2. Brief Description of the Related Art

Stress corrosion cracking (SCC) is a damaging process which is traced back to the simultaneous occurrence of tensile stress, a corrosive environment, and a material which is susceptible to it. The elimination or alteration of one of these three factors can frequently eliminate stress corrosion cracking itself, or reduce the susceptibility to it, and so constitutes a way of controlling stress corrosion cracking in practice.

Stress corrosion cracking is a sub-critical crack growth phenomenon which includes crack initiation at specific points, crack propagation and ultimately the final overload fracture. A failure as a result of stress corrosion cracking frequently occurs even in a seemingly moderate chemical environment with tensile stresses far below the yield point of the respective metal. Stress corrosion cracking therefore also continues to be the reason for significant machine failures.

In steam turbines, stress corrosion cracking occurs exclusively under wet steam conditions. Experiences from practice show that stress corrosion cracking occurs either in the fastening region of the blade (at the edges of the pin holes, at the fir-tree roots, or at the “straddle roots”) or at the transition from the rotor disks to the rotor shaft.

In the past, the widest variety of proposals have been put forward in order to prevent or at least to reduce stress corrosion cracking in steam turbines. In DE-A1 10 2004 028 395, it was proposed to reduce stress corrosion cracking by coating the parts of the steam turbine which are at risk with a noble metal (see also JP-A 60212603). For combating stress corrosion cracking, it is proposed in DE-A1 10 2006 013 139 to increase the yield point of the rotor in the region of the final turbine stage in relation to the previous stages. For stress minimization of stress corrosion cracking, EP-A1 2 143 884 describes providing the front side of the wheel disk in question of the rotor with a thermal barrier coating. JP-A 61149501 proposes injecting hydrazine into the steam in order to reduce oxygen in the steam to water. According to JP-A 61261605, the pH value of the steam is kept within a predetermined range in order to avoid stress corrosion cracking. All of these solution proposals are very costly either for production engineering reasons or for operational and technical reasons.

Finally, JP-A 60209602 proposes to dry wet steam (state C in FIG. 6) by throttling in a narrow gap (state D in FIG. 6) in order to guide the then dry steam through the rotor in the axial direction through gaps in each case between the rotor shaft and the shrunk-on rotor disks. Impulse rotors in monoblock construction or assembled construction, in which corresponding interspaces between shaft and wheel are not made available, cannot be protected against stress corrosion cracking in this way. In particular, only gaps between rotor and wheel disk in the case of assembled impulse rotors can be protected in this way, but not the remaining wheel disk contour.

SUMMARY

One of numerous aspects of the present invention relates to a method for operating a steam turbine with impulse rotor by which stress corrosion cracking in the region of the wheel disks can be combated in a simple and efficient manner, and also to a steam turbine useful for implementing the method.

Another aspect of the present invention includes that dry superheated steam is fed to the steam turbine, and that for reducing or for avoiding stress corrosion cracking on wheel disks of the impulse rotor which are exposed to the risk of wet steam, the dry superheated steam is used for purging of the wheel disks of the impulse rotor which are at risk.

Stress corrosion cracking (SCC) requires the influence of a liquid medium. From the thermodynamic point of view, steam wetness is formed in steam-water cycles upon entry of the expansion line into the so-called wet-steam region (passage through the Wilson line as the boundary line between dry superheated steam and wet steam). In order to reliably protect the impulse-rotor wheel disks against stress corrosion cracking, the steam which sweeps over them must be dry superheated. In order to achieve this, a dry superheated leakage steam flow is guided along the wheel disk towards the diaphragm packing.

Another aspect includes that the dry superheated steam is fed to the impulse rotor from an external steam source via a separate feed line. As an external steam source, for example, a high-pressure turbine section or intermediate-pressure turbine section or an external steam line is a possibility in this case. The external steam is then directed to stage 1 especially in front of the drilled wheel disk. Such a development would be expedient when the steam turbine was going to operate totally in the wet-steam region.

Another aspect includes that dry superheated steam is fed to the steam turbine via the inlet duct and, on account of the increasing expansion inside the steam passage, switches from a dry-steam region into a wet-steam region, and that for reducing or for avoiding stress corrosion cracking on wheel disks of the impulse rotor which are exposed to the risks of wet steam, a dry superheated leakage steam flow is tapped off from the dry superheated steam and used for purging of the wheel disks of the impulse rotor which are at risk.

In particular, the dry superheated leakage steam flow in this case is tapped off directly downstream of a first stage (N) of the impulse rotor which is located in the dry steam region and fed to the wheel disks of one or more subsequent stage(s).

It is especially simple if the dry superheated leakage steam flow is tapped off directly from the steam passage.

Another aspect includes that the feed of the dry superheated leakage steam flow is carried out via corresponding leakage steam holes or openings in the wheel disks.

In particular, for each wheel disk, a plurality of preferably axially oriented leakage steam holes or openings are arranged in a distributed manner over the circumference of the wheel disk for this purpose.

In particular, the steam turbine can be designed as a double-flow low-pressure steam turbine. The method, however, can also be used for single-flow low-pressure steam turbines. In this case, the leakage steam holes can also be expediently used for the purpose of axial thrust compensation.

A steam turbine embodying principles of the present invention comprises a plurality of stages which are arranged in series along a machine axis and with each of which is associated a wheel disk equipped with corresponding rotor blades, wherein the rotor blades of the individual stages project into a common axial steam passage in which steam enters via an inlet duct, and between the stages diaphragms are arranged in each case, wherein for reducing or for avoiding stress corrosion cracking on wheel disks which are at risk provision is made for means for feeding dry superheated steam to the wheel disks at risk, wherein the rotor of the steam turbine is designed as an impulse rotor, and the feeding means comprise a multiplicity of leakage steam holes or openings in the wheel disks.

According to another aspect, for feeding dry superheated steam, provision is made for a separate feed line which is connected to an external steam source.

A further aspect includes that, for each wheel disk, a plurality of preferably axially oriented leakage steam holes or openings are arranged in a distributed manner over the circumference of the wheel disk.

Another aspect includes that the steam turbine is designed as a double-flow low-pressure steam turbine.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention shall subsequently be explained in more detail based on exemplary embodiments in conjunction with the drawing. In the drawing:

FIG. 1 shows, in a detail, the longitudinal section through a flow of a double-flow low-pressure steam turbine with the dry steam purging of the wheel disks according to an exemplary embodiment of the invention with a leakage steam flow; and

FIG. 2 shows, in a view comparable to FIG. 1, another exemplary embodiment of the invention, in which via a separate feed line external dry steam is supplied for the purging.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Proposed solutions for avoiding stress corrosion cracking in the wheel disks of an impulse rotor of a steam turbine as described herein are aimed at eliminating the wet steam conditions as the cause of stress corrosion cracking in the region of the wheel disks. This can be achieved by dry superheated leakage steam being directed from stages of the steam turbine located upstream to stages located downstream which would otherwise be exposed to wet steam conditions. This is carried out in a particularly simple and efficient manner by specially dimensioned holes or openings being provided in the wheel disks and arranged in a distributed manner along the wheel circumference.

Shown in FIG. 1 is an exemplary embodiment for a steam turbine according to principles of the present invention. FIG. 1 in this case shows a detail of a flow of a double-flow low-pressure per se steam turbine with an impulse rotor. The steam turbine 10 includes an impulse rotor 11, which is rotatably supported around a machine axis 12, and has a multiplicity of stages N, N+1, N+2, N+3, N+4 (in general, N+x). Each of the stages N, N+1, N+2, N+3, N+4 is formed from a corresponding wheel disk 11 a-e, on the outer periphery of which are arranged the rotor blades 17 a-e of the turbine. Diaphragms 16 b-e are positioned between the wheel disks 11 a-e with the rotor blades 17 a-e. Rotor blades 17 a-e and diaphragms 16 b-e project radially into the steam passage 15 through which flows the steam which drives the turbine. The steam is introduced as dry superheated steam 14 into the steam passage 15 by an inlet duct 13. At the end of the inlet duct 13, provision is customarily made in front of the first stage N for a first group of guide vanes 16 a.

The dry superheated steam 14 which is introduced into the turbine by the inlet duct 13 is successively expanded in the steam passage 15 and after covering a certain axial distance (in the phase diagram) consequently changes from the dry-steam region DS into the wet-steam region WS which are separated from each other by the so-called Wilson line 19. In the wet-steam region WS, the wheel disks are exposed to stress corrosion cracking on account of the moisture. In order to ensure a dry environment for the wheel disks in question (in FIG. 1 these are the wheel disks 11 b and those following), preferably immediately after the first stage N a dry superheated leakage steam flow 18 is tapped from the still dry superheated main steam flow and guided radially inwards between rotor blades 17 a and diaphragm 16 b and around the inner periphery of the diaphragm 16 b in order to then impinge upon the wheel disk 11 b of the next stage N+1.

Provision is made in this wheel disk 11 b for the first (preferably axial) leakage steam holes 20 by which the tapped-off dry superheated leakage steam can enter the interspace between the wheel disks 11 b and 11 c of the two stages N+1 and N+2 and also ensure a dry environment there. Further leakage steam holes 21, 22 in further wheel disks 11 c and 11 d follow, guiding the dry superheated leakage steam into further interspaces of the wheel disks and preventing wet steam-induced stress corrosion cracking there. As a result of specific dimensioning of the leakage steam holes and of the diaphragm packings, a surplus of leakage steam is brought about (for the wheel disk 11 c, the surplus steam which sweeps over the two sides of the wheel is exemplarily identified by the designations 23 and 24 in FIG. 1), which sweeps over the inlet-side and outlet-side wheel disk before re-entering the main flow. The expansion of the main steam flow remains almost uninfluenced by it. By the same token, the overall efficiency of the cylinder remains almost uninfluenced or can even be increased due to the fact that suitable dimensioning of the leakage steam holes 21, 22 and 23 minimizes the re-entry of the leakage steam into the main flow with the efficiency loss which is linked to it.

Another exemplary embodiment of the invention is shown in FIG. 2. Here, the dry superheated steam 25 which is required for protection of the wheel disks is not tapped from the steam passage 15 but is supplied from an external steam source via a (schematically shown) separate feed line 26. An external steam source, for example a high-pressure turbine section or intermediate-pressure turbine section or an external steam line, is a possibility in this case. The external steam is then guided to stage 1 or N especially in front of the wheel disk 11 a which is provided with openings 27. Such a configuration would be expedient when the steam turbine was going to operate totally in the wet-steam region and all wheel disks would have to be protected.

Leakage steam holes of the above-described type can be provided in one or more wheel disks in order to create dry superheated steam conditions for desired wheel disks or regions of the rotor shaft located downstream, providing the mechanical integrity is still ensured even with the higher disk temperatures and with the presence of the holes (the disk temperatures decrease downstream, however). In the case of new rotors or replacement rotors within the scope of a retrofit process, these additional mechanical loads can be taken into account when designing the wheel disks in question. In the case of rotors which are already in use, the mechanical characteristics have to be redefined in order to prove the integrity of the rotor.

In summary, it can be said that (in an exemplary embodiment) a dry superheated leakage steam flow at the outlet of stage N is drawn from the main flow in front of the diaphragm of stage N+1 and guided along the wheel disk towards the diaphragm packing. The leakage steam holes 20 in stage N+1 effect a directed guiding of this dry superheated leakage steam flow in the direction of the following stage N+2. Without holes, this dry superheated leakage steam flow, after the diaphragm (guide vanes 16 b), would be returned again along the wheel disk of stage N+1 to the main flow in the steam passage 15 in front of the rotor blade row of stage N+1 and be mixed with the main flow. Without holes, the subsequent wheel disk of stage N+2 and those following would continue to be in the wet-steam region. The leakage steam holes and the diaphragm packings are dimensioned by calculation so that the leakage steam flow which passes through them corresponds at least to the leakage mass flow which is required for stage N+2 plus a small surplus which sweeps over the inlet-side and outlet-side wheel disk of stage N+1 before re-entering the main flow. This prevents induction of an additional moisture-laden leakage mass flow from the main flow in front of stage N+2. Depending upon stress state and temperature, the wheel disks of the subsequent stages N+3, N+4, etc, can also be designed with leakage steam holes in order to ensure a dry superheated steam state in the region of the wheel disks which are arranged there.

LIST OF DESIGNATIONS

-   Steam turbine (low-pressure with impulse rotor) -   11 Impulse rotor -   11 a-e Wheel disk -   12 Machine axis -   13 Inlet duct -   14, 25 Dry superheated steam -   15 Steam passage -   16 a Guide vane -   16 b-e Diaphragm -   17 a-e Rotor blade -   18 Leakage steam flow (dry superheated) -   19 Wilson line -   20, 21, 22 Leakage steam hole -   23, 24 Surplus steam -   26 Feed line -   27 Opening -   DS Dry-steam region -   N, N+1, . . . , N+4 Stage -   WS Wet-steam region

While the invention has been described in detail with reference to exemplary embodiments thereof, it will be apparent to one skilled in the art that various changes can be made, and equivalents employed, without departing from the scope of the invention. The foregoing description of the preferred embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. The embodiments were chosen and described in order to explain the principles of the invention and its practical application to enable one skilled in the art to utilize the invention in various embodiments as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto, and their equivalents. The entirety of each of the aforementioned documents is incorporated by reference herein. 

1. A method for operating a steam turbine having an impulse rotor, the impulse rotor having a multiplicity of stages arranged in series along a machine axis and with each of which is associated a wheel disk equipped with corresponding rotor blades, wherein the rotor blades of the individual stages project into a common axial steam passage in which steam enters via an inlet duct, diaphragms arranged between the stages in each case, and at least one of said wheel disks being exposed to a risk of stress corrosion cracking from wet steam, the method comprising: feeding dry superheated steam to the steam turbine; and purging said at least one wheel disk which is at risk with said dry superheated steam at a flow rate sufficient to reduce stress corrosion cracking on said at least one wheel disk which is exposed to the risks of wet steam.
 2. The method as claimed in claim 1, wherein feeding comprises feeding dry superheated steam from an external steam source via a separate feed line.
 3. The method as claimed in claim 1, wherein feeding comprises feeding dry superheated steam to the steam turbine via the inlet duct, and wherein increasing expansion of the superheated steam inside the steam passage switches said superheated steam from a dry-steam region to a wet-steam region, and further comprising: tapping a dry superheated leakage steam flow from the dry superheated steam and using dry superheated steam from said tapping in said purging.
 4. The method as claimed in claim 3, wherein: tapping comprises tapping a dry superheated leakage steam flow off directly downstream of a first stage of the impulse rotor which is located in said dry-steam region; and purging comprises supplying the dry superheated leakage steam flow from said tapping to at least one wheel disk of at least one downstream stage.
 5. The method as claimed in claim 3, wherein tapping comprises tapping the dry superheated leakage steam flow directly from the steam passage.
 6. The method as claimed in claim 1, wherein feeding comprises feeding a dry superheated leakage steam flow through leakage steam openings in the wheel disks.
 7. The method as claimed in claim 6, wherein each wheel disk comprises a plurality of leakage steam openings circumferentially distributed in each wheel disk.
 8. The method as claimed in claim 7, wherein the plurality of leakage steam openings are each axially oriented.
 9. The method as claimed in claim 1, wherein the steam turbine is a double-flow low-pressure steam turbine.
 10. A steam turbine comprising: an impulse rotor defining a machine axis; a multiplicity of stages arranged in series along the machine axis; a wheel disk associated with each stage, each wheel disk comprising rotor blades, wherein the rotor blades project into a common axial steam passage; an inlet duct leading to the common axial steam passage from which duct steam can enter into the steam passage; diaphragms arranged between the stages; and means for feeding dry superheated steam to wheel disks which are at risk of stress corrosion cracking, the means for feeding comprising a multiplicity of leakage steam openings in the wheel disks.
 11. The steam turbine as claimed in claim 10, wherein the means for feeding dry superheated steam comprises a separate feed line and an external steam source connected to the feed line.
 12. The steam turbine as claimed in claim 10, wherein the multiplicity of leakage steam openings comprises, for each wheel disk, a plurality of leakage steam openings circumferentially distributed in the wheel disk.
 13. The steam turbine as claimed in claim 12, wherein the plurality of leakage steam openings are each axially oriented.
 14. The steam turbine as claimed in claim 10, wherein the steam turbine comprises a double-flow low-pressure steam turbine. 